Drive apparatus

ABSTRACT

A hydrostatic transaxle with a vent assembly disposed in a port. The vent assembly may include a main fitting having a bottom member having an inner surface forming a floor and a fitting sidewall extending upwardly from the periphery to encircle the floor. A cap is secured to the sidewall and the fitting sidewall, cap and floor form a basin. A valve is disposed in the fitting sidewall. The assembly also includes cylindrical lower section extending downwardly from the bottom member to form a passage between the opening and a sump. A tower extends upwardly from the floor to the vent cap and surrounds an opening in the floor. A passage formed in the tower member to permit fluid flow between the opening and the basin.

CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No. 14/920,706, filed on Oct. 22, 2015, which claims the benefit of U.S. Provisional Pat. App. No. 62/067,334, filed on Oct. 22, 2014. These prior applications are incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

This application relates to a hydrostatic drive apparatus generally, and in particular, to a hydrostatic transaxle with a bypass mechanism and a brake mechanism. The transaxle is intended for use in driving a vehicle or other powered machine or apparatus.

SUMMARY OF THE INVENTION

An improved hydrostatic transaxle with a bypass mechanism and a brake mechanism is disclosed and described in detail herein. The transaxle can be mounted on a vehicle or other powered machine or apparatus to provide propulsion in cooperation with a prime mover.

A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth illustrative embodiments that are indicative of the various ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of a transaxle in accordance with the teachings herein.

FIG. 2 is a perspective view of a portion of the transaxle shown in FIG. 1, with certain components removed for clarity.

FIG. 3 is a perspective view of the main housing of the transaxle of FIG. 1.

FIG. 4 is a perspective view of the portion of the transaxle shown in FIG. 2, rotated 180° about the axis of a pump input shaft, with additional components removed for clarity.

FIG. 5 is a perspective view of the portion of the transaxle shown in FIG. 4, with additional components removed for clarity.

FIG. 6 is a perspective view of the portion of the transaxle shown in FIG. 5, rotated 180° about the axis of a motor output shaft, with additional components removed for clarity.

FIG. 7 is a perspective view of a portion of an alternate transaxle similar to the embodiment shown in FIG. 2, having alternate embodiments of cradle bearings, a swash plate, an inner brake puck and an outer brake puck.

FIG. 8 is a perspective view of another embodiment of a transaxle in accordance with the teachings herein.

FIG. 9 is a perspective view of a portion of the transaxle shown in FIG. 8, with certain components removed for clarity.

FIG. 10 is a bottom view of a portion of the transaxle shown in FIG. 8, with certain components removed for clarity.

FIG. 10A is another view of the portion of the transaxle as shown in FIG. 10, with additional components removed for clarity.

FIG. 11 is a top view of the portion of the transaxle shown in FIG. 10, rotated 90° about the axis of a pump input shaft, with additional components removed for clarity.

FIG. 12 is a perspective view of a portion of the transaxle shown in FIG. 8 with certain components removed for clarity.

FIG. 13 is an exploded perspective view of a filter assembly of a transaxle in accordance with the teachings herein depicting its cooperation with a pair of check valves.

FIG. 13A is an exploded perspective view of another embodiment of a filter assembly depicting its cooperation with an alternate pair of check valves.

FIG. 14 is a side view of a vent assembly of a transaxle in accordance with the teachings herein.

FIG. 15 is a cross-sectional view of the vent assembly shown in FIG. 14 along the line 15-15.

FIG. 16 is an exploded perspective view of the vent assembly shown in FIG. 14.

FIG. 17 is a top view of another embodiment of a vent assembly.

FIG. 18 is a cross-sectional view of the vent assembly of FIG. 17 along the line 18-18.

FIG. 19 is a perspective view of the base of the vent assembly of FIG. 17.

FIG. 20 is a perspective view of the cap of the vent assembly of FIG. 17.

FIG. 21 is a perspective view of another embodiment of a transaxle in accordance with the teachings herein.

FIG. 22 is a bottom view of a portion of the transaxle shown in FIG. 21, with certain components removed for clarity.

FIG. 23 is a perspective view of the differential assembly of the transaxle shown in FIG. 21.

FIG. 24 is a partially-exploded, perspective view of the differential assembly of FIG. 23, with certain components removed for clarity.

FIG. 25 is a partially-exploded, elevational view of the differential assembly of FIG. 23, with certain components removed and the housing shown cut away for clarity.

FIG. 26 is another partially-exploded, elevational view of the differential assembly of FIG. 23, with certain components removed and the housing shown cut away for clarity.

FIG. 27 is another partially-exploded, elevational view of the differential assembly of FIG. 23, with certain components removed and other components shown cut away for clarity.

DETAILED DESCRIPTION OF THE DRAWINGS

The description that follows describes, illustrates and exemplifies one or more embodiments of the invention in accordance with its principles. This description is not provided to limit the invention to the embodiment(s) described herein, but rather to explain and teach the principles of the invention in order to enable one of ordinary skill in the art to understand these principles and, with that understanding, be able to apply them to practice not only the embodiment(s) described herein, but also any other embodiment that may come to mind in accordance with these principles. The scope of the invention is intended to cover all such embodiments that may fall within the scope of the appended claims, either literally or under the doctrine of equivalents.

It should be noted that in the description and drawings, like or substantially similar elements may be labeled with the same reference numerals. However, sometimes these elements may be labeled with differing numbers or serial numbers in cases where such labeling facilitates a more clear description. Additionally, the drawings set forth herein are not necessarily drawn to scale, and in some instances proportions may have been exaggerated to more clearly depict certain features. As stated above, this specification is intended to be taken as a whole and interpreted in accordance with the principles as taught herein and understood by one of ordinary skill in the art.

FIG. 1 shows a first embodiment of a hydrostatic transaxle 120. A housing cover 123 is secured to a main housing 122 with fasteners 124. An axle 182 extends from main housing 122 and is supported in an axle horn 122 a at an inner bearing or journal location 122 b and an outer bearing or bushing location 122 c. A pump input shaft 129 also extends from main housing 122 and may have a pulley (not shown) fixed thereon capable of receiving a drive belt (not shown). A prime mover (not shown), such as an internal combustion engine or an electric motor, can supply motive force to the pump input shaft 129 of transaxle 120 by means of the belt and pulley combination. Transaxle 120 includes a block-lift bypass mechanism 160 having a bypass actuator 161 with an integrally-formed arm 161 a extending from main housing 122. A control arm 136 that ultimately controls the rotational speed and direction of axle 182 is fixed to a trunnion arm 134 (depicted in FIGS. 2 and 4) that also extends from main housing 122.

FIGS. 2 to 5 depict a transmission 121 and a reduction gear set 180 of transaxle 120. Transmission 121 includes an axial piston pump 130 which hydraulically communicates with an axial piston motor 140 through porting having two separate passageways (not shown) formed in a center section 150. Center section 150 is secured in main housing 122 by fasteners 151. Pump input shaft 129 extends through an opening 135 b in a swash plate 135 to the exterior of main housing 122. Pump input shaft 129 is rotatably supported in an input shaft bore 150 i of center section 150 and is further supported by a bearing (not shown) seated in an input shaft opening 122 f of main housing 122. A pump cylinder block 131 is engaged to pump input shaft 129. Pump cylinder block 131 has a set of pump pistons 131 a axially disposed therein and runs on a pump running surface 150 a of center section 150. Swash plate 135 controls the output of axial piston pump 130 in a known manner as a result of the arcuate motion of swash plate 135 when trunnion arm 134 is rotated by control arm 136. Alternatively, as is known, trunnion arm 134 may be rotated by other means, such as an electric actuator. This arcuate motion of swash plate 135 changes the angle of a pump thrust bearing 132, which is seated in swash plate 135, relative to the axial alignment of pump pistons 131 a to impart reciprocal motion to pump pistons 131 a when pump input shaft 129 and pump cylinder block 131 are rotationally driven by a prime mover (not shown), generating hydraulic fluid flow of variable displacement between axial piston pump 130 and axial piston motor 140.

A pair of guide ribs 135 a formed on swash plate 135 engages a pair of guide grooves 122 g formed in main housing 122 to guide the arcuate movement of swash plate 135. Guide ribs 135 a also interface with a pair of notches 137 a formed in a cradle bearing 137 to capture and move cradle bearing 137 along with swash plate 135. Cradle bearing 137 is in sliding contact with a cradle bearing mating surface 122 h formed in main housing 122 to each side of guide grooves 122 g. Pump input shaft 129 extends through an input shaft opening 137 b in cradle bearing 137. Input shaft opening 137 b is elongated similar to opening 135 b to provide clearance around pump input shaft 129 when swash plate 135 is moved within its range of motion. In the illustrated embodiment, the centerlines of guide ribs 135 a and corresponding guide grooves 122 g all lie on a plane that intersects the rotational axis of pump input shaft 129.

A motor cylinder block 141 of axial piston motor 140 has pistons 141 a axially disposed therein and runs on a motor running surface 150 b, which generally lies perpendicular to pump running surface 150 a. The flow of hydraulic fluid from axial piston pump 130 into pistons 141 a causes pistons 141 a to bear against a motor thrust bearing 142. Because motor thrust bearing 142 is positioned at a fixed angle relative to the axial alignment of pistons 141 a adjacent a motor displacement surface 122 m formed in main housing 122, motor cylinder block 141 is urged to rotate. Motor cylinder block 141 is slidingly engaged to a motor output shaft 145 at a motor shaft first end 145 a via splines or the like and causes motor output shaft 145 to rotate. Reduction gear set 180 is engaged to motor output shaft 145 at a motor shaft second end 145 b and is driven thereby. Motor output shaft 145 is rotatably supported solely by center section 150 in a cylindrical bore 150 j as shown in FIGS. 5 and 6. Bearings or the like (not shown) can be employed in cylindrical bore 150 j if needed.

A hydraulic circuit exists between axial piston pump 130 and axial piston motor 140 that is normally closed during operation of transaxle 120 with the exception of make-up hydraulic fluid from a sump 115, which can enter the circuit through a valve opening 155 b formed in a threaded plug 155 of one of a set of check valves 153 installed in center section 150 as shown in FIG. 6. Each check valve 153 contains a bleed passage 155 c in threaded plug 155 to reduce the harshness or immediacy of the transaxle's response to operator inputs at control arm 136, improving the ride quality of any vehicle equipped with transaxle 120. Check valves 153 communicate with the internal porting (not shown) of center section 150 by allowing make-up hydraulic fluid to be drawn through the check valve 153 that communicates with the low pressure or suction side of the hydraulic circuit, which may be one or the other of check valves 153 depending upon the direction of fluid circulation between axial piston pump 130 and axial piston motor 140, thereby compensating for fluid losses during operation. A filter, substantially similar to the filter 356 depicted in FIG. 10, can be attached to center section 150 in a known manner to remove contaminants from the hydraulic fluid entering through check valves 153, if needed, to extend the service life of transaxle 120.

Reduction gear set 180 includes a pinion gear 185 shown in FIGS. 2 and 5 supported on and driven by motor output shaft 145, a combination gear 186 having an inner gear 186 a drivingly engaged to an outer gear 186 b, and a final drive gear 187. Combination gear 186 is supported on a jack shaft 188 that is supported at one end by center section 150. Jack shaft 188 is supported at its opposite end at the housing joint by support pockets 122 s and 123 a formed in main housing 122 and housing cover 123, respectively. Final drive gear 187 is supported on and drives axle 182.

Axle 182 is journalled in axle horn 122 a at inner bearing location 122 b and is supported by an axle bushing 183 at outer bearing location 122 c. As shown in FIG. 2, axle bushing 183 includes fluid grooves and, additionally, fluid grooves (not shown) are formed inside axle horn 122 a through the journal area alongside axle 182 to allow sump fluid distribution throughout axle horn 122 a, including lubrication of an axle seal 181. Optionally, bearings or bushings or a combination of known bearing and bushing components can be used at inner bearing location 122 b and outer bearing location 122 c.

During assembly, axle 182 is installed through axle horn 122 a and through a splined hub 187 a of final drive gear 187 such that an inner end 182 a of axle 182 is positioned adjacent a thrust surface 122 d formed on main housing 122, as best shown in FIG. 3. Then, a C-clip 189 is installed in a groove 182 b formed on axle 182. Although other types of retaining clips other than C-clip 189 could be used in this assembly, C-clip 189 simply slides into groove 182 b and can therefore be installed without using a tool. Final drive gear 187 is then moved axially along a set of splines 182 c of axle 182 (as depicted in FIG. 4) toward axle thrust surface 122 d so that C-clip 189 is nested and retained in a C-clip recess 187 b formed in splined hub 187 a.

Shown in FIG. 4, a gear spacer 184 is then installed between final drive gear 187 and main housing 122 adjacent the side of final drive gear 187 opposite C-clip recess 187 b so that C-clip 189 remains nested and retained in C-clip recess 187 b. An extension 184 a of gear spacer 184 engages a spacer anti-rotation feature 122 e depicted as a slot formed in main housing 122 to prevent rotation of gear spacer 184. Through this arrangement of components, final drive gear 187 is properly positioned, axial movement of final drive gear 187 is restricted in both directions, axle 182 is retained in main housing 122, and axial movement of axle 182 is restricted in both directions.

As shown in FIGS. 3 and 5, mating features formed on center section 150 and main housing 122 ensure proper orientation of center section 150 at installation. Specifically, two alignment pockets 150 c formed concentrically about a first and a second center section fastener opening 150 d mate with two pilot bosses 122 i formed concentrically about threaded pockets 122 j to establish X-Y positioning of center section 150 in main housing 122. Planar mating surfaces 150 n and 122 w of these same two alignment pockets 150 c and two pilot bosses 122 i establish a Z axis position of center section 150. A third set of planar mating surfaces 150 k and 122 v, formed on center section 150 and main housing 122, respectively, add mounting stability and further establish the Z axis position of center section 150 and ensure perpendicularity of pump running surface 150 a to pump input shaft 129. Planar mating surfaces 150 n and 150 k are formed on center section 150 parallel to pump running surface 150 a. The Z axis in this description is an axis parallel to or equivalent to the rotational axis of pump input shaft 129. A third center section fastener opening 150 h is formed through planar mating surface 150 k, and a mating threaded pocket 122 k is formed through planar mating surface 122 v. Three center section mounting fasteners 151 oriented parallel to the rotational axis of pump input shaft 129 are installed through two openings 150 d and one opening 150 h to engage two threaded pockets 122 j and one threaded pocket 122 k, respectively, to secure center section 150 to main housing 122.

Referring to FIGS. 3, 4, 5 and 6, block-lift bypass mechanism 160 allows axle 182 to freely rotate when hydrostatic transaxle 120 is not under power. This allows an operator of a vehicle equipped with hydrostatic transaxle 120 to freely move the vehicle without powering it up, e.g. when servicing the vehicle. A bypass actuator 161, comprising a bypass arm 161 a and a bypass actuation rod 161 b, extends into main housing 122 with bypass actuation rod 161 b oriented generally parallel to pump input shaft 129. As shown, bypass arm 161 a and bypass actuation rod 161 b can be integrally formed portions of bypass actuator 161, though a separate arm and rod combination (not shown) may also be used in a known manner. O-ring grooves 161 d and 161 e are formed on actuation rod 161 b to receive a retention O-ring 165 and a sealing O-ring 166, respectively. Sealing O-ring 166 prevents fluid leaks by sealing around actuation rod 161 b in a conventional manner in an opening 122 t formed in main housing 122. Retention O-ring 165 abuts an internal land 122 u formed on main housing 122 to prevent outward movement of bypass actuator 161, thereby retaining bypass actuator 161 in main housing 122. This dual O-ring configuration allows bypass actuation rod 161 b, with O-rings 165 and 166 installed thereon, to be inserted into main housing 122 through opening 122 t. Upon insertion of bypass actuation rod 161 b, retention O-ring 165 immediately retains bypass actuator 161 when it passes through opening 122 t, thereby enabling simple assembly. Optionally, one or both of O-rings 165 and 166 can be integrally formed features of bypass actuation rod 161 b, thereby further simplifying assembly.

Bypass actuation rod 161 b is rotatably supported at its inner end 161 j in a bypass rod pivot opening or cradle 150 g of center section 150 as shown in FIG. 6. A U-shaped block-lift member 163 comprises a first block-lift leg 163 a having a first formed end 163 d, a second block-lift leg 163 b having a second formed end 163 e, and a pivot rod 163 c connecting first block-lift leg 163 a to second block-lift leg 163 b. Motor running surface 150 b is somewhat smaller than the diameter of motor cylinder block 141 to permit block lift member 163, and in particular block-lift leg 163 a and second block-lift leg 163 b, to contact motor cylinder block 141 at opposite sides of motor running surface 150 b. Pivot rod 163 c of block-lift member 163 is rotatably disposed in a pivot groove 150 e of center section 150. A cam form 161 c on the inner end 161 j of actuation rod 161 b is positioned adjacent to first formed end 163 d; as illustrated in FIG. 6, block-lift bypass mechanism 160 is in a disengaged state. When bypass actuator 161 is rotated in either a clockwise or counterclockwise direction to initiate engagement of block-lift bypass mechanism 160, cam form 161 c bears against first formed end 163 d, thereby forcing block-lift member 163 to rotate about the axis of pivot rod 163 c. A set of retention projections 163 f formed on each of formed ends 163 d and 163 e interface with a pair of arcuate surfaces 150 f formed on center section 150 to guide the rotation of block-lift member 163 while retaining pivot rod 163 c in pivot groove 150 e. When block-lift member 163 is rotated, first formed end 163 d and second formed end 163 e cooperatively contact and lift motor cylinder block 141 from motor running surface 150 b, breaking hydraulic fluid communication between motor pistons 141 a and fluid ports 150 m formed in motor running surface 150 b, and thus breaking the fluid communication between pump 130 and motor 140. Thus, motor output shaft 145, reduction gear set 180, and axle 182 are free to rotate without hydraulic resistance.

FIGS. 1, 2 and 3 best illustrate a brake mechanism 170. A brake shaft 172 is rotationally supported and sealed in an opening 122 p formed in main housing 122. When brake shaft 172 is rotated either clockwise or counterclockwise, via corresponding rotation of a brake arm 171 from an initial position to a rotated position, a cam form 172 a on brake shaft 172 forces a brake puck 174 against a brake rotor 176, which is mounted on pinion gear 185. Cam form 172 a also acts as a slot to locate brake puck 174 and prohibit movement of brake puck 174 in an axial direction along the axis of rotation of brake shaft 172. Pinion gear 185 is mounted on motor shaft second end 145 b of motor output shaft 145. As shown in FIG. 5, pinion gear 185 has a drive gear portion 185 a and a brake rotor mounting portion 185 b. Brake rotor 176 is slidingly mounted on brake rotor mounting portion 185 b of pinion gear 185. A second brake puck 175 is disposed in main housing 122 between brake rotor 176 and a thrust surface 122 r formed on main housing 122. Inner puck locators 122 n and outer puck locators 122 q are formed on main housing 122 to help guide and retain brake pucks 174 and 175, respectively. As brake shaft 172 is rotated further, additional axial movement of brake puck 174 forces brake rotor 176 into frictional engagement with brake puck 175 as brake puck 175 is forced against thrust surface 122 r, thereby braking motor output shaft 145. Additionally, brake puck 175 and thrust surface 122 r cooperatively serve as thrust elements for motor output shaft 145 to limit the movement of motor output shaft 145 in a direction along its rotational axis. As indicated in FIG. 1, a spring 173 is disposed about brake shaft 172 between brake arm 171 and main housing 122, bearing against each to bias brake mechanism 170 to a disengaged state. The bidirectional nature of brake mechanism 170 permits an external brake linkage (not shown) to actuate brake mechanism 170 in either rotational direction.

An alternate brake puck embodiment is illustrated in FIG. 7. A brake shaft 272 with brake pucks 274 and 275 are arranged approximately the same as in the previously described configuration. When brake shaft 272 is rotated clockwise or counterclockwise, frictional braking is applied to brake rotor 176 as in previously described brake mechanism 170. Brake pucks 274 and 275, however, have a greater contact area with brake rotor 176 for improved braking efficiency and lower heat generation. Additionally, ease-of-installation grooves 275 a and 275 b are provided on T-shaped brake puck 275. Groove 275 b provides clearance around motor shaft second end 145 b during assembly while groove 275 a engages a projection (not shown) formed on the transaxle main housing (not shown) that guides brake puck 275 into position during assembly and then properly positions brake puck 275 in relation to motor shaft second end 145 b when brake puck 275 is fully installed.

An alternate swash plate embodiment in the form of a swash plate 235 and a pair of clip-on cradle bearings 237 is also illustrated in FIG. 7. Pump input shaft 129 extends through an opening 235 b formed in swash plate 235. A pair of projections 235 c formed on swash plate 235 engages a pair of openings 237 c formed on each cradle bearing 237 such that cradle bearings 237 are locked in place on swash plate 235 to move along with swash plate 235. As in the first embodiment, the centerlines of a pair of guide ribs 235 a and corresponding pair of guide grooves (not shown) formed in the transaxle main housing (not shown) all lie on a plane that intersects the rotational axis of pump input shaft 129.

FIGS. 8 through 12 depict an embodiment of a hydrostatic transaxle 320 that varies from the previously described embodiments in terms of the manner in which its bypass actuation rod 361 b is retained in a main housing 322, and the manner in which its integral bypass arm 361 a interacts with the main housing 322 to limit rotation of the bypass actuation rod 361 b. Hydrostatic transaxle 320 also varies from the previously described embodiments in terms of the interaction of its main housing 322 and housing cover 323, the retention of cradle bearings 337 by its swash plate 335, a bushing retention feature (lip) 350 w formed in the motor running surface 350 b of its center section 350, additional support for loads induced at its jack shaft 388, and the details of its brake mechanism 370. The remaining components of hydrostatic transaxle 320 are substantially similar in form and function to those of the preceding hydrostatic transaxle embodiments and will not be further described herein.

Hydrostatic transaxle 320 comprises a housing cover 323 secured to a main housing 322 with fasteners 324. Integral locating pins 322 x formed along the perimeter of main housing 322 interact with pin alignment openings 323 b formed in housing cover flange 323 c to minimize sealant smearing during the joining of main housing 322 and housing cover 323. Unless otherwise described herein, main housing 322 and housing cover 323 are substantially similar in form and function to main housing 122 and housing cover 123, respectively. An axle 382 extends from main housing 322 and is supported in an axle horn 322 a at an inner bearing or journal location 322 b and an outer bearing or bushing location 322 c. A pump input shaft 329 also extends from main housing 322 and may have a pulley (not shown) fixed thereon capable of receiving a drive belt (not shown). A prime mover (not shown), such as an internal combustion engine or an electric motor, can supply motive force to the pump input shaft 329 of transaxle 320 by means of the belt and pulley combination. Transaxle 320 includes a block-lift bypass mechanism 360 having a bypass actuator 361 with an integrally-formed arm 361 a extending from main housing 322. A control arm 336 that ultimately controls the rotational speed and direction of axle 382 is fixed to a trunnion arm 334 (depicted in FIGS. 9 and 11) that also extends from main housing 322.

Center section 350, shown in more detail in FIGS. 11 and 12, includes a pump running surface 350 a and a motor running surface 350 b formed on center section 350 generally perpendicular to each other. An axial piston pump 330 is disposed on the pump running surface 350 a. A pump cylinder block 331 of axial piston pump 330 is engaged to and driven by pump input shaft 329. Pump cylinder block 331 has a set of pump pistons 331 a axially disposed therein and runs on the pump running surface 350 a of center section 350.

The interaction of the set of pump pistons 331 a with a pump thrust bearing 332 disposed in swash plate 335 to create hydraulic displacement in response to arcuate movement of the swash plate 335 is as previously described for axial piston pump 130 and will not be further detailed herein. Similarly, a pair of guide ribs 335 a formed on swash plate 335 engages a pair of guide grooves 322 g formed in main housing 322 to guide the arcuate movement of swash plate 335 in the manner previously described for swash plates 135 and 235, respectively. Retention of each of a pair of clip-on cradle bearings 337 on the external, arcuate surface of swash plate 335 is accomplished through the interaction of a pair of projections 335 c formed on swash plate 335 with a pair of openings 337 c formed at opposing ends of cradle bearing 337 in the manner previously described for swash plate 235. However, swash plate 335 provides additional retention capability for the cradle bearing 337 disposed adjacent trunnion arm 334 through use of a pair of integrally-formed tabs 335 d, as depicted in FIG. 11. As installed, cradle bearings 337 are placed in sliding contact with cradle bearing mating surfaces 322 h formed adjacent opposing sides of guide grooves 322 g in main housing 322. Overall, guide ribs 335 a and the corresponding guide grooves 322 g formed in main housing 322 each have a centerline that lies on a plane intersecting the rotational axis of pump input shaft 329.

A hydraulic motor 340 having an axial piston motor cylinder block 341 is disposed on motor running surface 350 b. Motor cylinder block 341 has a set of pistons 341 a axially disposed therein. Axial piston motor 340 is in fluid communication with axial piston pump 330 via porting having two separate passageways (not shown) formed in center section 350 between pump running surface 350 a and motor running surface 350 b. The flow of hydraulic fluid from axial piston pump 330 into pistons 341 a causes pistons 341 a to bear against a motor thrust bearing 342. Because motor thrust bearing 342 is positioned at a fixed angle relative to the axial alignment of pistons 341 a adjacent a motor displacement surface 322 m formed in main housing 322, motor cylinder block 341 is urged to rotate. Motor cylinder block 341 is slidingly engaged to motor output shaft 345 and causes motor output shaft 345 to rotate.

Motor output shaft 345 is rotationally supported in a cylindrical bore 350 j of center section 350. A bushing retention lip 350 w is integrally formed with motor running surface 350 b to restrain axial movement of a bushing (not shown) pressed into cylindrical bore 350 j that rotationally supports motor output shaft 345. At a first end (not shown), motor output shaft 345 is drivingly engaged to motor cylinder block 341. At a second end 345 b, motor output shaft 345 engages pinion gear 385 to drive reduction gear set 380 and axle 382 in the manner previously described for reduction gear set 180 and axle 182, respectively, and will not be further detailed herein. Thus, the combination of axial piston pump 330, axial piston motor 340 and center section 350 forms a transmission 321 drivingly engaged to a reduction gear set 380 disposed in sump 315 that powers axle 382.

Referring to FIGS. 9, 10, 10A, and 11, center section 350 of transaxle 320 is oriented and mounted in main housing 322 in a manner substantially similar to that of center section 150, with the addition of a fourth set of planar mating surfaces 350 q, 322 aa formed on center section 350 and a strengthening rib 322 bb of main housing 322, respectively, These respective surfaces cooperate to bear loads induced at the jack shaft pocket 350 r formed in center section 350 that provides support to jack shaft 388 at a first end. This fourth set of planar mating surfaces 350 q, 322 aa also cooperates to establish a Z axis position, as defined below, of center section 350. It should be noted that jack shaft 388 is supported and captured at its second end by a pair of cooperating support pockets 322 s, 323 a formed in main housing 322 and housing cover 323, respectively, along the housing joint.

In the manner previously detailed for center section 150, two alignment pockets 350 c formed concentrically about a first and a second center section fastener opening 350 d mate with two pilot bosses 322 i formed concentrically about threaded pockets 322 j to establish X-Y positioning of center section 350 in main housing 322. The planar mating surfaces 350 n, 322 w of these same two alignment pockets 350 c and two pilot bosses 322 i, respectively, establish a Z axis position of center section 350. The Z axis in this description is an axis parallel to or equivalent to the rotational axis of pump input shaft 329. A third set of planar mating surfaces 350 k, 322 v formed on center section 350 and main housing 322, respectively, add mounting stability and further establish the Z axis position of center section 350. The planar mating surfaces 350 n, 350 k are formed on center section 350 parallel to pump running surface 350 a, helping to ensure the perpendicularity of pump running surface 350 a relative to pump input shaft 329. A third center section fastener opening 350 h is formed through planar mating surface 350 k, and a corresponding threaded pocket 322 k is formed through planar mating surface 322 v. Three center section mounting fasteners 351 oriented parallel to the rotational axis of pump input shaft 329 are installed through the center section fastener openings 350 d, 350 h to engage threaded pockets 322 j, 322 k, respectively, to secure center section 350 to main housing 322.

Additional functionality imparted to center section 350 includes a pair of threaded check valve ports 350 s, a pair of filter pin locating pockets 350 u, and a threaded filter screw pocket 350 t, all formed on the side of center section 350 opposite pump running surface 350 a. Referring to FIGS. 10, 12, and 13, each check valve port 350 s communicates with one of the two separate passageways (not shown) formed in center section 350 between pump running surface 350 a and motor running surface 350 b. A pair of check valves 353 is disposed in the pair of check valve ports 350 s to provide make-up hydraulic fluid to the closed hydraulic loop between axial piston pump 330 and axial piston motor 340. Check valves 353 comprise a threaded plug 355 having a valve seat 355 a disposed about a first end of a valve opening 355 b that passes longitudinally through threaded plug 355 and has a hexagonal form for use with a hex tool during installation of check valves 353 in check valve ports 350 s. These threaded plugs 355 may optionally have a bleed passage (not shown) as described for check valves 153, depending on the desired ride quality of any vehicle equipped with transaxle 320. A check ball 352 that interacts with valve seat 355 a is retained by a set of flexible ribs 354 a in cage 354 of check valve 353. Cage 354 can be made of a flexible polymer such as glass filled nylon (e.g. a PA46 glass filled nylon), permitting the check ball to be inserted through flexible ribs 354 a during assembly. An error proofing tab 354 b prevents cage 354 from being installed in check valve port 350 s in a reverse orientation, as it acts to prevent installation of threaded plug 355 when installed improperly.

An alternate embodiment of a check valve 553 is illustrated in FIG. 13A, wherein cage 554 is symmetrically designed to allow reversible installation in check valve port 350 s of center section 350, eliminating the need for an error proofing tab. In the manner previously described for check valve 353, a check ball 552 is sized to sealingly interact with a valve seat 555 a disposed about a first end of a valve opening 555 b longitudinally formed in a threaded plug 555. Check ball 552 may be pressed through either end of cage 554 to be retained therein by a plurality of ribs 554 a, wherein the material composition of cage 554 may be that of a flexible polymer such as a glass filled nylon. Threaded plug 555 may optionally have a bleed passage 555 c whose diameter may be varied to produce a desired ride quality in any vehicle equipped with transaxle 320. It should be noted that the longitudinal length of the combined components of check valves 353 and 553 will be the same for purposes of installation in a given port, such as check valve port 350 s, whereby the relative lengths of their cages 354, 554 and threaded plugs 355, 555 can differ, but are dimensioned to result in the same overall length.

Referring again to FIGS. 10, 12 and 13, a filter 356 may be installed on center section 350 to remove contaminants from hydraulic fluid entering the closed hydraulic loop through one or the other of check valves 353. The check valve 353 in communication with the low pressure or suction side of the closed hydraulic loop may permit make-up fluid to pass through its valve opening 355 b as check ball 352 is displaced from valve seat 355 a by differential pressure on each side of check valve 353. Filter 356 comprises a filter base 357 and a filter cover 358 whose respective side walls are sealingly joined when fastener 359, in this instance a threaded screw, affixes filter 356 to center section 350 at threaded filter screw pocket 350 t. A pair of filter locating pins 357 a integrally-formed on filter base 357 mate with filter pin locating pockets 350 u on center section 350 to properly align the openings 357 c formed on filter base 357 with each of the check valves 353 installed in center section 350. A deformable lip 357 b surrounds each of the openings 357 c to sealingly engage the external face of each valve plug 355 when fastener 359 is properly torqued. As best shown in FIG. 10, filter cover 358 has a filter screen 358 b to permit the intake of hydraulic fluid from sump 315 through check valves 353 and a reinforced bridge structure 358 a to accept the loading of fastener 359 and also deflect the stream of high pressure hydraulic fluid escaping through an optional bleed passage (not shown) in valve plug 355, protecting filter screen 358 b from damage. A magnet 348 may be disposed in magnet pocket 357 d to collect any fine metal shavings circulating in the hydraulic fluid to improve the service life of critical components of transaxle 320.

An alternate embodiment of a filter 556 is depicted in FIG. 13A, wherein the filter screen 557 d has been relocated to the filter base 557. As affixed to center section 350 in the manner previously described for filter 356, filter screen 557 d is oriented at the top of filter 556, reducing the introduction of entrained air into the closed hydraulic loop of center section 350 and improving hydraulic efficiency. Filter cover 558 is now formed as a solid cover that sealingly engages filter base 557 in the manner previously described for filter 356, and further provides reinforcement to accept the loading of a fastener, such as fastener 359, and deflects the stream of high pressure hydraulic fluid escaping through an optional bleed passage 555 c in threaded plug 555. The form and function of a pair of filter locating pins 557 a and a pair of deformable lips 557 b surrounding each of the openings 557 c of filter base 557 is substantially similar to that of filter base 357 and will not be further detailed herein. It should be noted that the filters 356, 556 and check valves 353, 553 described herein can be alternately paired with each other to work in combination with center section 350.

Referring to FIGS. 8, 9, 11 and 12, transaxle 320 includes an alternate embodiment of a block-lift bypass mechanism 360 having a U-shaped block-lift member 363 and a bypass actuator 361. Bypass mechanism 360 functions in substantially the same manner previously described for bypass mechanism 160, but varies in terms of the means by which its bypass actuation rod 361 b is retained in main housing 322, and the manner in which its bypass arm 361 a interacts with main housing 322 to limit rotation of the bypass actuation rod 361 b.

Bypass mechanism 360 has block-lift member 363 moveably disposed adjacent motor cylinder block 341 and bypass actuator 361. Bypass actuator 361, including a bypass arm 361 a and a bypass actuation rod 361 b, extends into main housing 322. In this embodiment, bypass arm 361 a is integrally formed with bypass actuation rod 361 b, though it could be detachably joined thereto in a known manner, and bypass actuation rod 361 b is oriented generally parallel to pump input shaft 329. A first end 361 j of bypass actuation rod 361 b can be formed as a cam section 361 c that permits bypass actuator 361 to be rotated in a clockwise direction or a counterclockwise direction in actuating bypass mechanism 360.

Bypass arm 361 a, which is formed to effect rotation of bypass actuation rod 361 b in cooperation with a mechanical linkage (not shown), includes one or more rotation stop tabs, 361 f and 361 i for example, which, along with the edges 361 g, 361 h of bypass arm 361, engage various features on main housing 322 to prevent over-rotation of bypass actuation rod 361 b. This ensures that the amount of block-lift achieved (i.e. hydraulic bypass) builds to a maximum when the stop tabs 361 f, 361 i or edges 361 g, 361 h engage corresponding features on main housing 322 to prevent an over-rotation condition where hydraulic resistance may still be present in the closed hydraulic loop between hydraulic pump 330 and hydraulic motor 340. Such resistance hinders manual movement of an unpowered vehicle equipped with transaxle 320. It is important to note that as depicted in FIG. 8, transaxle 320 is a right-hand unit, meaning that it would be mounted on the right-hand side of a vehicle with its input shaft 329 oriented toward the front of the vehicle. Correspondingly, a mirror-image left-hand unit (not shown), would be mounted on the left-hand side of a vehicle such that its input shaft would also be oriented toward the front of the vehicle. Stop tab 361 f lies below the plane of bypass arm 361 a to engage a boss 322 dd formed on main housing 322 that stop tab 361 i passes over during counterclockwise rotation of bypass actuator 361, as stop tab 361 i lies within the plane of bypass arm 361 a. During clockwise rotation, block-lift bypass is maximized when edge 361 g contacts input shaft boss 322 z. Conversely, for a left-hand transaxle unit (not shown), stop tab 361 i engages a housing boss somewhat taller than boss 322 dd during clockwise rotation of bypass actuator 361, and edge 361 h contacts the input shaft boss during counterclockwise rotation of bypass actuator 361. Thus, bypass actuator 361 may be used in both right-hand and left-hand transaxle units.

Bypass actuation rod 361 b is shown in FIG. 12 with first end 361 j rotatably supported in a bypass rod pivot opening or cradle 350 g formed in center section 350 and positioned adjacent block-lift member 363. A second end of the bypass actuation rod 361 b extends through a bypass actuation rod opening 322 t formed in main housing 322. An O-ring groove 361 e is shown as formed on bypass actuation rod 361 b and structured to receive a sealing O-ring (not shown). The sealing O-ring can aid in the prevention of fluid leaks by sealing around bypass actuation rod 361 b in a conventional manner in opening 322 t. In addition, one or more retention tabs 361 d can be formed on bypass actuation rod 361 b, where a pair of exemplary retention tabs 361 d is illustrated in FIG. 12, to prevent outward movement of bypass actuator 361, thereby retaining bypass actuation rod 361 b in main housing 322. Retention tabs 361 d are flexible in nature to compress during passage through opening 322 t and expand after insertion into main housing 322, acting therein against bypass actuator retention surface 322 u to ensure retention. The O-ring and retention tab configuration allows bypass actuation rod 361 b to easily be inserted into main housing 322 through opening 322 t during assembly.

In FIGS. 11 and 12, bypass mechanism 360 includes a block-lift member 363 having a first block-lift leg 363 a with a first formed end 363 d, a second block-lift leg 363 b with a second formed end 363 e, and a pivot rod 363 c connecting first block-lift leg 363 a to second block-lift leg 363 b. Pivot rod 363 c of block-lift member 363 can be rotatably disposed in a pivot groove 350 e formed on center section 350 and have a locating protrusion 363 g on pivot rod 363 c structured to engage a gap between locating or retention ribs 350 p on center section 350. Cam form 361 c on the first end of bypass actuation rod 361 b is positioned adjacent first formed end 363 d. Cam form 361 c of bypass actuation rod 361 can bear against first formed end 363 d of first block-lift leg 363 a to force block-lift member 363 to rotate about an axis of pivot rod 363 c. A retention projection 363 f is formed on each of first formed end 363 d and second formed end 363 e where each of retention projections 363 f interfaces with a corresponding surface 350 f formed on center section 350 to retain pivot rod 363 c in pivot groove 350 e. Corresponding surface 350 f of various embodiments can be an arcuate surface. Block-lift member 363 straddles motor output shaft 345 and contacts motor cylinder block 341 in multiple locations when lifting motor cylinder block 341 from motor running surface 350 b.

As illustrated in FIG. 12, block-lift bypass mechanism 360 is in a disengaged state. Rotation of bypass actuation rod 361 b causes block-lift member 363 to contact motor cylinder block 341 in at least two places to thereby lift motor cylinder block 341 away from motor running surface 350 b. When bypass actuator 361 is rotated in either a clockwise or counterclockwise direction to initiate engagement of block-lift bypass mechanism 360, cam form 361 c bears against first formed end 363 d, thereby forcing block-lift member 363 to rotate about the axis of pivot rod 363 c. Retention projections 363 f formed on each of formed ends 363 d and 363 e interface with corresponding surfaces 350 f formed on center section 350 to guide the rotation of block-lift member 363 while retaining pivot rod 363 c in pivot groove 350 e. When block-lift member 363 is rotated, first formed end 363 d and second formed end 363 e cooperatively contact and lift motor cylinder block 341 from motor running surface 350 b. Formed ends 363 d, 363 e and their respective block-lift legs 363 a, 363 b form gradual curves along their lengths to ensure that contact with motor cylinder block 341 occurs on both sides of motor cylinder block 341 near a line that bisects motor output shaft 345 and lies parallel to the axis of pivot rod 363 c, reducing wear on components by lifting motor cylinder block 341 in an axial direction with respect to the axis of motor output shaft 345. This lift breaks hydraulic fluid communication between motor pistons 341 a and a set of fluid ports 350 m formed in motor running surface 350 b; and more generically, it breaks the fluid communication between axial piston pump 330 and axial piston motor 340. Thus, motor output shaft 345, reduction gear set 380, and axle 382 are free to rotate without hydraulic resistance.

The brake mechanism 370 of transaxle 320, as illustrated in FIGS. 8, 9, 10 and 10 a, has substantially similar form and function to the previously described brake mechanism embodiments, particularly that of FIG. 7, but comprises additional error-proofing features provided for its outer puck 375 and a revised brake arm 371 and brake shaft 372 engagement. As compared to outer puck 275, outer puck 375 of the present embodiment has an additional groove 375 c for error-proofing purposes during installation. Grooves 375 a and 375 c cooperate with a pair of projections 322 y formed on main housing 322 to ensure proper orientation of outer puck 375 during assembly of transaxle 320. It should be understood that a plurality of corresponding grooves and projections can be utilized for this purpose. Clearance around the second end 345 b of motor output shaft 345 is provided by a groove 375 b on the opposite face of outer puck 375 from grooves 375 a and 375 c. A pair of outer puck locators 322 q formed on main housing 322 help guide and retain outer brake puck 375, along with a thrust surface 322 r formed on main housing 322. Unlike the inner puck locators 122 n of brake mechanism 170 (a pair of projections), the additional width of inner puck 374 is accommodated by inner puck locators 322 n (a tab and an inner housing wall of main housing 322).

As compared to the engagement means of brake arm 171 and brake shaft 172, wherein a roll pin 177 or the like not only retains brake arm 171 on brake shaft 172, but also bears the torque load associated with actuating brake mechanism 170, the external end of brake shaft 372 is formed with a single D-shaped cross-section 372 b to more robustly engage a brake arm 371, which has a corresponding single D-shaped opening 371 a. A fastener, such as exemplary rivet 377, merely retains brake arm 371 on brake shaft 372 and prevents axial movement along the axis of brake shaft 372. The torque load associated with actuating brake mechanism 370 is borne by the single D-shape engagement. The biasing of brake mechanism 370 to a disengaged state by a spring, such as torsion spring 373, is as previously described for brake mechanism 170 and will not further be described herein. Similarly, the function of brake mechanism 370 through use of a brake rotor 376 slidingly disposed about a pinion gear 385 and further disposed between and selectively engaged by an inner puck 374 and an outer puck 375 upon rotation of a brake shaft 372 having a cam form 372 a on the end proximate to inner puck 374, which also serves to retain inner puck 374, is as previously described for brake mechanism 170 and will not be further detailed herein.

As shown in FIGS. 8, 10 and 10A, transaxle 320 is outfitted with a vent assembly 325 located in a threaded port 322 cc formed at or near the highest point of main housing 322. Referring to FIGS. 14, 15 and 16, vent assembly 325 has a flow path formed through it to prevent the buildup of case pressure in transaxle 320 as the temperature of its hydraulic fluid increases under operating conditions, venting trapped air and in extreme cases, expanded hydraulic fluid, to protect the integrity of various shaft seals found in transaxle 320. A corresponding function of vent assembly 325 is to prevent the intrusion of water and other contaminants, as can occur during washing of a vehicle equipped with transaxle 320. Vent assembly 325 includes a main body 326, a valve 327, and a vent cap 328.

Vent cap 328 is generally formed as a flattened dome, though other external shapes can be suitable. A spray shield 328 b is shown extending from the internal surface of vent cap 328 to form a circular wall. A locking groove 328 a is formed near the periphery of the internal surface of vent cap 328.

Main body 326 is generally formed as a cylindrical fitting having an externally threaded lower portion 326 b and a tool form upper portion 326 a radially disposed about a valve seat 326 e to form an open center cavity 326 p. A primary passage 326 c runs throughout the length of main body 326, and a downwardly sloped flange 326 i extends radially about main body 326 between its upper and lower portions. Primary passage 326 c is open at a first, lower end and terminates at a second, upper end having a reduced diameter valve retention aperture 326 d and a plurality of smaller valve passages 326 f radially disposed thereabout, each passing through to valve seat 326 e. A drain groove 326 g is disposed about valve seat 326 e within center cavity 326 p and lies below the surface of valve seat 326 e. One or more drain passageways 326 h provide fluid communication between drain groove 326 g, and consequently center cavity 326 p, and the exterior of main body 326. When vent assembly 325 is installed in threaded port 322 cc of main housing 322, an O-ring (not shown) is utilized to insure a proper seal, and an appropriate tool is used to engage tool form 326 a to first turn down main body 326 to an appropriate torque. As illustrated, tool form 326 a is an external hex-form, such that an open wrench or socket may be used to install main body 326. Other specialized tool forms and corresponding tools are contemplated within the scope of this application, including an internal hex-form such that a key wrench may be applied to install main body 326.

A flexible umbrella valve 327 is disposed in valve retention aperture 326 d such that its retention bead 327 a lies within primary passage 326 c and its sealing flange engages valve seat 326 e to close off the plurality of valve passages 326 f. The pressure rating of the particular umbrella valve 327 utilized can be selected to prevent shaft seal leakage and meet the requirements of the specific application to which transaxle 320 is applied. As case pressure in transaxle 320 builds with increasing temperature during operation and reaches a value in excess of the pressure rating of umbrella valve 327, sealing flange 327 b is lifted off of valve seat 326 e, opening valve passages 326 f to atmospheric pressure and allowing air, and in extreme cases hydraulic fluid, to be vented though valve passages 326 f.

Vent cap 328 is snapped onto main body 326 after installation of umbrella valve 327 in valve retention aperture 326 d to complete the installation of vent assembly 325. Locking groove 328 a engages the periphery of downwardly sloped flange 326 i to loosely join the components of vent assembly 325, permitting the passage of air and liquids, such as water or hydraulic fluid, therebetween. As assembled, spray shield 328 b surrounds tool form 326 a to form a baffle to thwart intrusion of water or other fluids. In the event that intrusive water does reach center cavity 326 p, it will collect in drain groove 326 g, pass through drain passages 326 h, follow along downwardly sloping flange 326 i and then proceed out of the vent assembly 325 between the loose engagement of flange 326 i and locking groove 328 a. The release of air and/or hydraulic fluid by umbrella valve 327 will result in the discharge of the respective fluids from vent assembly 325 by means of the same pathway described for the drainage of water from vent assembly 325.

An additional embodiment of a vent assembly 525 that may be utilized with a transaxle, such as transaxle 320, is illustrated in FIGS. 17, 18, 19, and 20. Like vent assembly 325, vent assembly 525 comprises a main body 526, a valve 527, and a vent cap 528. Vent assembly 525, however, benefits from an expanded internal volume that acts as a catch basin for hydraulic fluid and hinders its discharge through valve 527. Vent assembly 525 also benefits from having its tool form 528 a disposed on the external surface of vent cap 528 such that vent assembly 525 can be preassembled prior to installation on a transaxle. Though shown as a hex-form, tool form 528 a can be formed in a variety of shapes to cooperated with known tools.

Main body 526 is formed as a basin-shaped fitting having an externally threaded cylindrical lower section 526 b disposed about a primary passage 526 c open at a first, lower end. Primary passage 526 c opens at a second, upper end into a circular basin 526 g having a floor 526 q that slopes down to primary passage 526 c for drainage purposes and a sidewall 526 r. Passing through sidewall 526 r are a valve retention aperture 526 d and a pair of valve passages 526 f positioned proximate to, and above, valve retention aperture 526 d. A flexible umbrella valve 527 is mounted in valve retention aperture 526 d in the manner previously described for umbrella valve 327, selectively sealing valve passages 526 f at a valve seat 526 e formed on the external surface of sidewall 526 r. A spray guard 526 i formed external to sidewall 526 r just below valve retention aperture 526 d cooperates with a spray guard protrusion 528 c formed internal to vent cap 528 to protect umbrella valve 527 from water intrusion. Concentrically positioned atop sidewall 526 r is a chamfer 526 j adapted to cooperate with a cylindrical wall 528 b internal to vent cap 528 to capture an O-ring 539, sealing the joint therebetween. Rising from the floor 526 q of basin 526 g and concentrically positioned about the second, upper end of primary passage 526 c is a central tower 526 a having a plurality of anti-rotation slots 526 m that engage cooperating anti-rotation ribs 528 e internal to vent cap 528. Basin passage 526 h through central tower 526 a permits communication between basin 526 g and primary passage 526 c. A plurality of alignment slots 526 k above chamfer 526 j cooperate with a like number of alignment ribs 528 d formed internal to vent cap 528 to correctly align main body 526 and vent cap 528 for joinder. A plurality of snap-lock protrusions 526 n along the external periphery of main body 526 engage a like number of snap-lock ramps 528 f formed along the internal periphery of vent cap 528 to join the component parts of vent assembly 525.

Vent assembly 525 performs in the manner previously described for vent assembly 325 as temperatures rise during operating conditions, permitting the venting of expanding air to atmosphere through valve passages 526 f and the intervening space between main body 526 and vent cap 528, while improving the retention of hydraulic fluid expelled from the housing of a transaxle by means of basin 526 g and its communication with primary passage 526 c via basin passage 526 h.

FIGS. 21 through 27 depict an embodiment of a hydrostatic transaxle 420 that primarily varies from the previously disclosed embodiments in that its transmission 421 drives both a left-side axle 482L and a right-side axle 482R through a differential assembly 490. Correspondingly, main housing 419 comprises a left-side axle horn 419 a and a right-side axle horn 419 b to provide rotational support for axles 482L, 482R, respectively, in the manner previously described for axle horns 122 a, 322 a. Main housing 419 is sealingly closed by housing cover 323 with a plurality of fasteners 324 and integral locating pins 419 x as described for the joinder of main housing 322 and housing cover 323.

It will be understood that transmission 421 is substantially similar in form and function to transmission 321, and as such, will not be further detailed herein. Any revisions to the pump thrust bearing grade or any addition of a valve plate to the pump running surface to accommodate higher system pressures attendant with two-axle transaxles will be understood to lie within the scope of this disclosure. Similarly, though transmission 421 is shown as incorporating filter assembly 356, it will be understood that an alternate filter assembly, such as filter assembly 556 may be considered within the scope of this disclosure. But for the incorporation of a differential-accommodating final drive gear or bull gear 491, reduction gear set 480 may also be considered substantially similar in form and function to reduction gear sets 180, 380, and as such, will not be further detailed herein. Any revisions to the specific gear reduction ratios to limit hydraulic system pressures will be understood to lie within the scope of this disclosure.

FIGS. 23 through 27 detail differential assembly 490 and its assembly. Generally speaking, differential assembly 490 is a necessary addition to a two-axle transaxle driven by a single transmission, such as illustrated by transaxle 420 and transmission 421. During vehicle turns, the axle associated with the larger, outside turn radius must rotate at a greater rate than the axle associated with the smaller, inside turn radius. Differential assembly 490 permits this accommodation.

In general terms, differential assembly 490 comprises a pair of opposing planet bevel gears 492 whose rotational axes are collinearly disposed and a pair of opposing axle bevel gears 494 whose rotational axes (and correspondingly, those of left-side axle 482L and right-side axle 482R) are collinearly disposed. The rotational axes of these gear sets intersect in a perpendicular orientation, as each planet bevel gear 492 engages each axle bevel gear 494 in a quadrilateral engagement pattern.

Bull gear 491 has various openings 491 a, 491 b and recesses 491 c to accommodate the components of differential assembly 490 in this quadrilateral engagement pattern. Each of a pair of openings 491 a is configured to accept a pin 493 upon which a planet bevel gear 492 is rotationally supported, the pin having a flat end 493 a at each end, which when properly situated in opening 491 a, prevents its rotation. Each opening 491 a also accommodates the planet bevel gear 492 itself and serves as the gear's thrust surface under load where each of the planet bevel gears 492 and the axle bevel gears 494 tend to push out axially, expanding the quadrilateral engagement pattern. Opening 491 b accepts the proximal ends 482 a of the axles 482L, 482R which provide rotational support for bull gear 491. A pair of recesses 491 c disposed about opening 491 b on the opposing faces of bull gear 491 provide accommodation in the differential stack-up for a radial protrusion 494 a formed on the face of the axle bevel gears 494 to ensure retention of a C-clip 489 that retains axle bevel gear 494 on its respective axle 482L, 482R.

FIGS. 25 through 27 depict the sequencing of assembly for differential assembly 490. Each axle 482L, 482R is rotationally supported near its proximal end 482 a by a rotational support opening 419 d formed in main housing 419. An optional bushing (not shown) may line rotational support opening 419 d, depending on vehicle application. Adjacent to the proximal end 482 a of each axle 482L, 482R is a C-clip retaining groove 482 b and a set of splines 482 c adapted to engage a corresponding set of splines formed on the interior of axle bevel gear 494. Axle bevel gear 494 is first slipped onto splines 482 c, after which C-clip 489 is inserted in C-clip retaining groove 482 b. Axles 482L, 482R are moved outboard until each axle bevel gear 494 encounters an inner wall of main housing 419 adjacent to rotational support openings 419 d to provide space for insertion of bull gear 491 between axle bevel gears 494. This correspondingly locates C-clip 489 within a recess 494 b formed in the face of axle bevel gear 494 about which radial protrusion 494 a is disposed, further ensuring that C-clip 489 will not become dislodged from C-clip retaining groove 482 b. Next, bull gear 491, with planet bevel gears 492 and pins 493 installed, is inserted between axle bevel gears 494. Axles 482L, 482R are then moved inboard to produce a gear mesh between planet bevel gears 492 and axle bevel gears 494. Correspondingly, the radial protrusions 494 a of each axle bevel gear 494L, 494R are located within the corresponding recesses 491 c formed on the opposing faces of bull gear 491, but with appropriate tolerances, do not physically engage the recesses 491 c. As shown in FIG. 27, the proximal ends 482 a of axles 482L, 482R now reside within bull gear opening 491 b, to provide rotation support for bull gear 491 during differential operation. A pair of axle spacers 484 are then inserted between axle bevel gears 494 and main housing 419 to set the stack-up of differential assembly components. Each axle spacer 484 has a yoke 484 b disposed about its respective axle 482L, 482R and a protrusion 484 a that is captured in an anti-rotation slot 419 e between a pair of axle spacer retention ribs 419 c. Each axle spacer yoke 484 b also serves as a thrust surface for its respective axle bevel gear 494 upon installation.

During straight-line operation of a vehicle equipped with transaxle 420 and differential assembly 490, each axle 482L, 482R is rotated at the same rate as bull gear 491 transfers rotational energy from reduction gear set 480 to each axle 482L, 482R through the stack-up of planet bevel gears 492 and axle bevel gears 494, where each axle bevel gear 494 turns at the same rotational rate as bull gear 491. As a vehicle proceeds through a turn, the one of the axles making the larger radius turn will rotate at a greater rotational rate than bull gear 491, while the other of the axles making the smaller radius turn will rotate at a rate less than that of bull gear 491.

While one or more specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications, combinations of disclosed elements, and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of any appended claims and any equivalent thereof 

What is claimed is:
 1. A hydraulic drive apparatus, comprising: a housing forming an internal sump; a hydraulic drive mechanism disposed in the internal sump; at least one axle engaged to and driven by the hydraulic drive mechanism, the at least one axle extending out of the housing; a vent port formed in a portion of the housing and extending between an external surface of the housing and the internal sump; and a vent assembly disposed in the vent port, the vent assembly comprising: a main fitting having a bottom member having an inner surface forming a floor, an outer surface opposite the inner surface and a periphery, and a fitting sidewall extending upwardly from the periphery to encircle the floor; a vent cap secured to an upper portion of the fitting sidewall and comprising a top surface member and a cap sidewall extending downwardly from the top surface member, wherein the fitting sidewall, the vent cap and the floor form a basin; a valve disposed in the main fitting; an opening formed in the floor; a cylindrical lower section extending downwardly from the outer surface of the bottom member to form a primary passage between the opening and the internal sump, wherein the cylindrical lower section is disposed in the vent port; a tower member extending upwardly from the floor to the vent cap, the tower member surrounding the opening; and a passage formed in the tower member to permit fluid flow between the opening and the basin.
 2. The hydraulic drive apparatus of claim 1, wherein the valve is a flexible umbrella valve disposed in a valve retention orifice disposed in the fitting sidewall, and further comprising a plurality of vent apertures formed in the fitting sidewall adjacent the valve retention orifice.
 3. The hydraulic drive apparatus of claim 1, further comprising a tool form formed on an external top surface of the vent cap.
 4. The hydraulic drive apparatus of claim 1, wherein the tower member engages an internal surface of the vent cap to prevent rotation of the vent cap with respect to the main fitting.
 5. The hydraulic drive apparatus of claim 4, further comprising a plurality of anti-rotation structures formed on the internal surface of the vent cap to engage a plurality of anti-rotation slots formed on the tower member.
 6. The hydraulic drive apparatus of claim 5, wherein the passage formed in the tower member forms one of the plurality of anti-rotation slots.
 7. The hydraulic drive apparatus of claim 1, wherein the floor is sloped from the fitting sidewall down to the opening.
 8. The hydraulic drive apparatus of claim 1, wherein a top surface of the fitting sidewall includes a chamfer to cooperate with an internal cylindrical wall of the vent cap.
 9. The hydraulic drive apparatus of claim 8, further comprising a lip formed adjacent the chamfer, the lip having a plurality of alignment slots formed thereon, each of the plurality of alignment slots cooperating with one of a plurality of alignment tabs formed on the vent cap.
 10. The hydraulic drive apparatus of claim 1, wherein the valve is disposed in the fitting sidewall and the cap sidewall covers an external portion of the valve.
 11. The hydraulic drive apparatus of claim 10, wherein the floor is sloped from the fitting sidewall down to the opening.
 12. The hydraulic drive apparatus of claim 10, further comprising a first plurality of locking mechanisms on the main fitting and a second plurality of locking mechanisms formed on the cap sidewall, wherein each of the first plurality of locking mechanisms cooperates with one of the second plurality of locking mechanisms to snap-lock the vent cap to the main fitting.
 13. The hydraulic drive apparatus of claim 1, further comprising a differential engaged to and driven by the hydraulic drive mechanism, and wherein the at least one axle consists of a pair of axles driven by the differential.
 14. A vent assembly for use with a hydraulic drive apparatus having a housing forming an internal sump, the vent assembly comprising: a main fitting having a bottom member having an inner surface forming a floor, an outer surface opposite the inner surface and a periphery, and a fitting sidewall extending upwardly from the periphery to encircle the floor; a vent cap secured to an upper portion of the fitting sidewall and comprising a top surface member and a cap sidewall extending downwardly from the top surface member, wherein the fitting sidewall, the vent cap and the floor form a basin; a valve disposed in the main fitting; an opening formed in the floor; a cylindrical lower section extending downwardly from the outer surface of the bottom member to form a primary passage between the opening and the internal sump, wherein the cylindrical lower section is capable of being disposed in a vent port of the housing; a tower member extending upwardly from the floor to the vent cap, the tower member surrounding the opening; and a passage formed in the tower member to permit fluid flow between the opening and the basin.
 15. The vent assembly of claim 14, wherein the valve is disposed in the fitting sidewall and the cap sidewall covers an external portion of the valve.
 16. The vent assembly of claim 15, further comprising a first plurality of locking mechanisms on the main fitting and a second plurality of locking mechanisms formed on an internal surface of the cap sidewall, wherein each of the first plurality of locking mechanisms cooperates with one of the second plurality of locking mechanisms to snap-lock the vent cap to the main fitting.
 17. The vent assembly of claim 16, further comprising a plurality of anti-rotation structures formed on an internal surface of the vent cap to engage a plurality of anti-rotation slots formed on the tower member.
 18. The vent assembly of claim 17, wherein the floor is sloped from the fitting sidewall down to the opening. 